The ultrasonic echolocation calls produced by bats are typically narrowband frequency modulated signals that may be described as whistles or sweeps. Conventionally, two well-known techniques have been used to analyze the echolocation calls of bats.
One technique for analyzing echolocation calls is based on zero crossing analysis where the instantaneous frequency of the echolocation call can be determined precisely by measuring the time period between one or more zero crossing points in the original time-domain signal. This method can produce very high resolution information about the frequency modulation of the call through time. However, because zero crossing techniques measure the frequency component of the signal with the greatest amplitude, it is possible that the analysis will sometimes favor a harmonic of the signal or that the signal becomes undetectable in the presence of noise. Additionally, echoes present in the signal are difficult to isolate from the original signal due to the lack of amplitude information available to zero crossing techniques.
The second technique for analyzing echolocation calls is based on the Fourier transformation in which blocks of digitized samples from the time domain signal are transformed into frequency bins representing the power spectrum. Spreading the signal into several frequency components has the advantage of isolating the harmonic components of a signal and isolating the signal from noise present in other frequency bands to greatly improve overall detection. However, Fourier techniques have limited temporal and frequency resolution relating to the block size and are unable to directly produce the instantaneous and precise frequency information possible with zero crossing techniques.